This has less to do with co-developing than custom-ordering, but it's the same strategy to me, that is, playing one OEM against another. It's just that a different company kowtows every year.
The Nexus 7 is a much more restrictive Android device than any other. According to Brian Klug of Anandtech, not only did Google omit microSD expansion (~$40 for 64GB card), they killed USB mass storage, too. USB-OTG is strictly for keyboards and such.
Therefore you're locked into this "tablet built for GooglePlay", in their own words.
It's like the smart-chip they put inside inkjet printers a few years ago to discourage third-party ink.
This is a business model where the device is subsidized by Google, rather than a free gift you seem to think.

According to Google's Rubin 'When it gets sold through the Play store there's no margin, it just basically gets (sold) through.'
http://www.knowyourmobile.com/blog/1455823/google_says_theres_no_margin_on_nexus_7.html
Google hopes to sell you content and take your personal info to show ads to make money. Basically 'spoiled the market' for other vendors.